1. Field of the Invention:
This invention relates to a novel optical information recording medium and the recording of information thereon. More particularly, the present invention relates to an information recording medium, preferably in the form of a disk or in a tape format, suitable for use with optical recording and playback apparatus, with the information layer of the recording medium comprising a chromophore/polymer composition. In particular, the polymer and/or chromophore of the composition are chemically bound in order to improve compatibility.
2. Description of the Prior Art:
Optical recording methods in which light from a laser is focused upon the surface of a recording medium with sufficient intensity to cause a detectable change in the physical characteristics of the surface material have been proposed. Among these methods is the establishment of an information pattern of pits. In such methods, the information representative pattern of pits may be formed in the surface of the recording medium by suitably controlling the intensity of the focused light in accordance with the information to be recorded while relative motion is established between the recording medium and the focused light spot.
The recording medium, of course, is one of the key elements in any optical information storage system. The commercial viability of the recording medium depends upon such technical parameters as the sharpness in recording and playback of the information, i.e., a high signal to noise ratio. Dyes and pigments have accordingly been employed in information layers, often to enhance the sensitivity of the recording layers at the particular wavelength of the laser being used, which results in a much sharper recording and playback of information.
For example, Spong, U.S. Pat. No. 4,097,895, describes a recording medium which comprises a light reflecting material, such as aluminum or gold, coated with a dye-containing light absorbing layer, such as fluorescein, which is operative with an argon laser light source. The thickness of the light absorbing layer is chosen so that the structure has minimum reflectivity. An incident light beam then ablates, vaporizes or melts the dye-containing light absorbing layer, leaving a hole and exposing the light reflecting layer. After recording at the wavelength of the recording light, maximum contrast between the minimum reflectance of the light absorbing layer and the reflectance of the light reflecting layer exists.
Carlson, in U.S. Pat. No. 3,475,760, discloses a system for directly recording information in a thermoplastic film as a deformation by using a high energy laser scanning beam of small diameter. It is further disclosed that the sensitivity of the films for laser film deformation recording can be enhanced by the addition of pigments or dyes which exhibit a high absorption at the laser wavelength. Erasure of the film deformation is accomplished by recording over the information to be erased using a similar laser beam but with a much smaller scan line spacing, preferably so as to provide overlap of the scan lines.
Other U.S. patents which disclose the use of a light absorbing dye in the recording layer include U.S. Pat. Nos. 4,412,231 and 4,446,223. The former patent discloses using a mixture of dyes having different light absorbing wavelengths so that the resulting recording layer has a light absorptivity of 80% or more at all the wavelengths in the range of from 400-900 nm. The latter patent discloses an optical information recording element comprising a support coated with a layer of an amorphous composition, which composition comprises a binder and an oxoindolizine or oxoindolizinium dye.
In a paper entitled "Single Wavelength Optical Recording in Pure, Solvent Coated Infrared Dye Layers" by Gravesteijn, Steenbergen and van der Veen, experiments on the use of certain dyes for optical recording for digital and video applications at GaAlAs laser wavelengths are reported. The paper was presented at the Proceeding of the SPIE, "Optical Storage Media", volume 420, June 6-10, 1983. The specific dyes discussed in the paper are squarylium dyes and pentamethine dyes. It is further suggested that solubility in organic solvents can be greatly increased by the introduction of t-butyl groups into thiopyrylium end groups.
The use of dyes in conjunction with optical recording media comprising a styrene oligomer is disclosed in the article by Kuroiwa et al appearing in the Japanese Journal of Applied Physics, Vol. 22, No. 2, February 1983, pp. 348-343. Among the dyes and pigments discussed as being useful is a copper phthalocyanine pigment. Phase separation and incompatibility between the dyes and oligomers are noted in the article as being problems in the use of dyes for optical information media.
The use of other metal phthalocyanine dyes in optical recording media is disclosed, for example, in U.S. Pat. No. 4,458,804. Note also, U.S. Pat. No. 4,492,750, which discloses the use of specific naphthalocyanine compounds in optical recording media. The film-coating properties of such dye materials, however, have been generally found to be relatively poor, the read out signal/noise (S/N) ratio relatively poor and tending to fluctuate depending on the particular portion of the layer, and the S/N ratio of the read-out deteriorating significantly after repeated irradiations of the read-out light.
Horiguchi et al, U.S. Pat. No. 3,637,581, discloses chromogen-bonded polymers, with the chromogen possibly being a metal phthalocyanine. The suitability and use of such products in optical mass data storage applications, however, are not disclosed therein.
Thus, while dyes or pigments have been employed in the information storage layers of optical recording media due to their excellent absorption properties, problems are encountered with regard to the application of the dyes or pigments in a stable layer. The addition of dyes to film-forming polymers due to limited solubility of the dye in the polymer and the tendency of the dye/polymer mixture to phase separate over time, as noted above, are severe problems which need to be overcome. Indeed, the higher the pigment or dye concentration, the more likely such problems are encountered. Yet, it is desired to increase the dye concentration in the information layer so as to increase the sensitivity of the medium, the recording rate possible and the S/N ratio upon read-out. Indeed, it is necessary for the dye to be uniformly distributed in the polymer matrix, at a concentration level to insure the presence of dye in every volume exposed to the laser radiation.
The search for an improved information storage medium comprising a dye or pigment composition overcoming the aforementioned problems is thereby continuously ongoing. What is desired is a recording layer material which of course exhibits a high extinction coefficient, but which also eliminates the problem of dye migration and hence phase separation over time. Excellent stability with respect to thermal, actinic and oxidative degradation is also a desirable feature.
Accordingly, it is a major object of the present invention to provide a novel and improved recording medium which comprises a chromophore in the information layer.
It is yet another object of the present invention to provide a novel optical recording medium which allows for ready application of the chromophore layer to form a stable information layer, while still exhibiting excellent absorption properties.
Still another object of the present invention is to provide a novel recording medium which contains a chromophore in the information layer, yet for which the problem of dye migration and hence phase separation over time frequently encountered in dye/polymer mixtures is eliminated.
These and other objects, as well as the scope, nature and utilization of the invention, will be apparent to those skilled in the art from the following description and the appended claims.